This invention relates to transmission-type liquid crystal display units having an edge-type backlight.
As shown in FIGS. 3(a) and 3(b), conventional transmission-type liquid crystal display units having an edge-type backlight include a liquid crystal cell 10 consisting of two glass substrates 11 and 12 and a liquid crystal 13 sealed between the glass plates 11 and 12 with a seal member 14, and two polarizing plates 15 and 16 on the outer surfaces of the glass substrates 11 and 12, respectively.
The liquid crystal display unit, being a transmission-type display unit in which light is applied from below the liquid crystal cell, further comprises a light source 17 which is, in this case, a fluorescent lamp mounted on one side of the display unit, a light-conducting plate 18 such as an acrylic plate for conducting light from the light source to the entire lower surface of the display, and a reflecting sheet 19 to prevent light from the light source from passing away from the light-conducting plate 18.
The light-conducting plate 18 has a light-receiving end face 20 which extends perpendicular to the display surface of the liquid crystal cell 10, i.e., perpendicular to the light-emitting surface of the light-conducting plate 18. The light source 17 has a single line contact with the light-receiving end face of the light-conducting plate 18. In the liquid crystal display unit shown in FIG. 3(a), the thickness of the light-conducting plate 18 is equal to the diameter of the fluorescent lamp 17. In the liquid crystal display unit shown in FIG. 3(b), the thickness of the light-conducting plate 18 is half the diameter of the fluorescent lamp 17.
Light from the fluorescent lamp 17 enters the light-conducting plate 18 through the light-receiving end face 20 and is transmitted through the light-conducting plate 18 by repeated internal reflections. During such reflections, part of the light passes through the surface of the light-conducting plate 18 to enter the liquid crystal cell 10, thus permitting an illuminated display of the liquid crystal image.
FIG. 4 is an enlarged view showing the light-receiving end face 20 of the light-conducting plate 18 of FIG. 3(a). A light beam L1 from the fluorescent lamp 17 incident on the light-receiving end face 20 of the light-conducting plate 18 at an angle .theta. is divided into a light beam L2 reflected by the end face 20 and a light beam L3 transmitted into the light-conducting plate 18. The transmitted light beam L3 contributes to the backlight for the liquid crystal display while the reflected light beam L2 is useless.
Thus, as represented in FIG. 4, when a light beam from the fluorescent lamp 17 is applied to the light-receiving end face 20 of the light-conducting plate 18 at an angle .theta., the portion of the light beam reflected by the end face 20 is not useful in the display unit. At the contact line P between the lamp 17 and the end face 20, where .theta.=90.degree., the proportion of light in the reflected light beam L2 is close to zero. However, it should be noted that there is only one such contact line with the light-conducting plate 18 and therefore there is a high loss of light from the fluorescent lamp 17. As a result, in conventional backlighted liquid crystal display units, the display may not be bright enough to be seen easily. In order to overcome this difficulty, it is necessary to use a larger fluorescent lamp providing a higher light output to take into account the loss of light, with the result that the size of the liquid crystal display unit is correspondingly increased.